The present invention relates to a method for creating an oxide coating on certain metals wherein the oxide coating is highly resistant to corrosion and ionic leaching. More particularly, this invention relates to a method for creating a protective coating on the surface of austenitic stainless steel or nickel alloy steel. The oxide coating is both resistant to corrosion and to leaching of molecules from the steel into material that is in contact with the metal.
Austenitic stainless steel and nickel alloy steel are commonly used for piping, vessels, and equipment used in processes in which the purity of the material being processed is a critical consideration. Such steels are also used in processes in which a strong solvent or other corrosive material is present. Many such processes are carried out at elevated temperatures. The presence of very pure materials, strong solvents, or corrosive materials, particularly at elevated temperatures, makes prevention of corrosion of the steel and/or control of infusion of various contaminating components from the steel difficult. Examples of processes for which prevention of corrosion, control of infusion, or both are critical considerations include chemical, food, pharmaceutical, and semiconductor processing.
Components used for such processes are generally polished to eliminate small surface protrusions from which ions may leach into the material being processed, or which may provide locations at which corrosion may begin. The oxide coating that naturally forms on the surfaces of austenitic stainless steels and nickel alloy steel is inadequate in many applications to prevent corrosion or unacceptable leaching of ions from the steel into material in contact with steel, particularly when the components are irregularly shaped or contain welds.
The oxide film that naturally forms on austenitic stainless steel and on nickel alloy steel contains both iron and iron oxides as well as chromium and chromium oxides. Oxide films that have a high ratio of chromium-to-iron have resistance to leaching that is superior to that of films having a lower chromium-to-iron ratio. Existing methods for enhancing the oxide film are not able to reliably create oxide films on austenitic stainless steel and nickel alloy steel components which have acceptable performance or durability in difficult applications. This is particularly true when the existing methods are applied to components having surface irregularities such as crevices of sharp angles and those formed by welding.